Some devices support the notification aggregation feature, and whether a notification is aggregated depends on the APP.
For your reference, the details about the reason are as follows:
Notifications that have been aggregated, or those within an aggregation group, do not support individual deletion. To delete all notifications in an aggregated group, tap on the aggregated notification card to enter the aggregation notification list, where you'll find an option at the bottom to [Clear All]. You can tap [Clear All] to remove all notifications in that group.